Quantum physics is a theory that abruptly breaks with classical physics, and offers us a revolutionary and often counter-intuitive thinking on the world around us. In this course we have a closer look at the underlying concepts and applications of this theory. The two-level system will be introduced on the basis of the quantum spin of particles. Central are systems with two or more particles, such as atoms and molecules, and the methods to describe and manipulate them, for instance with light.